Scientists have recently raised alarm over the possibility that some inks used for tattoos contain cancer-causing chemicals.

To make matters worse, some pigments come as small particles called nanoparticles that could easily enter the bloodstream and accumulate in organs such as the spleen and kidneys. These organs filter impurities and their failure may ultimately increase the risk of cancer.

But should we be worried? Let's look at the evidence.

Tattoo ink

There is no doubt some of the chemicals in tattoo ink have been associated with cancer.

The red colours can contain mercury which causes cancer in rats and growth problems in babies of exposed women. But no definite evidence exists of linking mercury exposure in humans to cancer.

The greens and blues contain cobalt, which has been found to cause cancers in animals - however, the risk in humans needs to be explored further.

One component of black ink, benzo(a)pyrene, is a potent cancer-causing chemical and has been linked to skin cancers in petroleum workers.

Last year, Jorgen Serup, Professor of Dermatology from Copenhagen University, reported that 13 of 21 tattoo inks commonly used in Europe contained cancer-causing chemicals. Even the tattoo ink manufacturers admit that 5% of their products contain these types of chemicals.

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But even if these inks do contain components which have the potential to cause cancer, we still don't know whether people who have tattoos are exposed to enough of them to actually cause cancer.

It may be that very large doses are needed, well beyond the exposure from tattooing, or perhaps only small doses are required over time. This is an important fact which needs further research before the link between tattooing and cancer can be confirmed.

The other issue is the nanoparticles.

There is nothing inherently dangerous about particles being tiny. While some people are concerned that nanoparticles in sunscreens could more easily pass through tissues into the blood stream, the available evidence shows they pose no risk.

Tattoo dyes, of course, are injected into the deeper layer of the skin. The issue here is less about the nanoparticle make-up and more about exposure to cancer-causing chemicals. But we still don't know whether enough chemical gets into the body to cause harm.

Further evidence could easily be collected by observing if people with tattoos have more cancers than those without. The problem here is that developing cancer is a process that often takes decades, so sufficient time between the tattoo and the recording of the cancer must be allowed.

Check out pics from the International Tattoo and Art Expo earlier this year:

During that time, people will have been exposed to multiple cancer causing agents in the environment. And, unless the association between cancer and tattoos is very strong, it may be difficult to tease out the tattoo as the cause of the cancer.

One review of the available literature found reports of 50 skin cancers in tattoos. The authors believed that was low compared to the number of tattoos (it has been estimated that one in five people in the UK have a tattoo) and thought it was coincidental, rather than the tattoo causing the cancer.

Tattoos may, however, make it difficult to detect skin cancers early and the tattoo may be obscured after surgery.

How should we respond?

It's important for regulators, often of the cosmetics industry under which these products commonly fall, to be aware of the chemicals that increase the risk of cancer. And it would prudent for manufacturers to remove these chemicals from the inks, particularly as they claim that they make up such a small percentage of the products.

If you're contemplating getting a tattoo, it's worth asking about the chemicals in the inks and avoiding those with the potential to cause cancer, if that's possible, even though the link with cancer needs further research.

But like all of the other major lifestyle risk factors for cancer - smoking, obesity, alcohol and sun protection - individuals must weigh the risk and benefit for themselves, a decision often influenced by the dictates of fashion.

* Ian Olver is CEO of Cancer Council Australia and a medical oncologist.